In recent years, semiconductor LSIs, chip components, and others have been increasingly made smaller in size and lower in stature, and the pitch between terminals has been increasingly made narrower. Ceramic substrates on which these members are to be mounted have also been required, for example, to be made smaller in size and lower in stature, and be made higher in precision. Hitherto, as a dielectric ceramic configured in a ceramic substrate, alumina has been mainly used. The firing temperature of alumina is as high as a temperature of 1300 to 1600° C.; thus, a high melting point metal such as W or Mo is used for the material of electrodes used in wiring lines, such as transmission line paths, on a ceramic substrate. However, W, Mo or the like is large in electric resistivity and in loss in a high-frequency range, so that the metal is not usable for components for which low-resistance wiring is required, or components for high frequencies. Thus, it has been desired to use a low-resistance metal such as Cu, Ag or Au. However, Cu, Ag, Au or the like is low in melting point, and thus a dielectric ceramic has been required to be capable of being sintered together with an electrode material at a low temperature of about 800 to 1000° C. Moreover, the dielectric constant of alumina is as large as about 10, so that in a transmission line path thereof, the transmission of high-frequency signals is delayed. In order to decrease the delay, dielectric ceramics have been desired which are relatively small in dielectric constant.
As a dielectric ceramic which is small in dielectric constant and can be sintered at low temperature, Patent Document 1 suggests an inorganic composition including at least one selected from alumina, cordierite, mullite and others, borosilicate based glass, and anorthite crystal. This dielectric ceramic is produced to include alumina in a proportion of 12 to 59.6% by mass of the whole of the ceramic, at least one selected from cordierite, mullite and others in a proportion of 10 to 30% by mass thereof, borosilicate based glass in a proportion of 18 to 69.6% by mass thereof and anorthite crystal in a proportion of 1 to 40% by mass thereof, where the whole of components be 100% by mass. However, the inorganic composition may cause, in a producing process thereof, for example, inconveniences that a B (boron) compound included in borosilicate based glass damages firing-furnace material, and that in the process of producing the inorganic composition into a ceramic sheet, the B compound is dissolved in water or alcohol and this compound segregates when the resultant workpiece is dried.
As a method for yielding a dielectric ceramic made dense at 900° C. without using borosilicate based glass, Patent Document 2 suggests a method of: calcining a raw material powder including oxides of Al, Si, Sr, Bi, Na, K, Cu and Mn to cause these oxides to react with each other to vitrify the oxides partially; pulverizing the resultant into fine particles of about 1 μm size; making the resultant powder into a low-temperature-sintering material; mixing this low-temperature-sintering material with cordierite or some other inorganic filler material that has been pulverized into fine particles of about 1 μm size at a prescribed ratio; shaping the mixture; and then firing the shaped body.